1. Field of the Invention
This invention relates to a radiation image read-out apparatus wherein a radiation image, which has been stored on a stimulable phosphor sheet, is read out from the stimulable phosphor sheet, and an image signal representing the radiation image is thereby obtained. This invention particularly relates to a radiation image read-out apparatus, which has a function for detecting the location of a pattern in the radiation image, which pattern corresponds to dust clinging to the stimulable phosphor sheet. This invention also relates to a method for displaying a radiation image, wherein a visible image is reproduced from an image signal, which has been obtained by detecting light emitted by a stimulable phosphor sheet, on which a radiation image has been stored, in proportion to the amount of energy stored thereon during its exposure to radiation.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, a-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as the human body. A radiation image of the object is thereby stored on the stimulable phosphor sheet. The stimulable phosphor sheet is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used during the reproduction of the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT) display device, or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain the desired image density, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT display device.
In the radiation image recording and reproducing systems which use stimulable phosphor sheets, if dust clings to part of the surface of a stimulable phosphor sheet, the image information at said part will be lost in a visible image reproduced from an image signal, which has been detected from the stimulable phosphor sheet. Additionally, the pattern corresponding to dust and information in the radiation image, which information is related to a diagnosis, or the like, cannot be discriminated from each other. Therefore, an accurate judgment, e.g. an accurate diagnosis of an illness, cannot be made from the visible image.
For example, an X-ray image of the mamma of a human body is stored on a stimulable phosphor sheet, and an electric signal representing the X-ray image is detected from the stimulable phosphor sheet. The X-ray image is then reproduced as a visible image from the electric signal and displayed on a CRT display device, or the like. A diagnosis is then made from the visible image or to whether the human body does or does not have mammary cancer. In cases where the human body has mammary cancer, white dot-like patterns, which correspond to calcified parts and have a diameter of approximately 300 .mu.m, appear in the X-ray image. Therefore, by way of example, the diagnosis is made based on the presence or absence, the number, the density, or the like, of such white dot-like patterns appearing in the X-ray image. However, in cases where dust clings to the stimulable phosphor sheet, patterns corresponding to the dust also appear as white dots in the visible image which is reproduced from the image signal detected from the stimulable phosphor sheet. It is difficult for the patterns corresponding to the dust to be discriminated from the patterns corresponding to the calcified parts. Accordingly, there is the risk that, even if no calcified part is present in the human body, the human body is diagnosed incorrectly as having mammary cancer.
By way of example, non-luminescent impurities are often included in the stimulable phosphor, which is used as a raw material for constituting the stimulable phosphor layer of a stimulable phosphor sheet. Also, dust is often included in the stimulable phosphor layer in the course of making a stimulable phosphor sheet. In such cases, small defects, which cannot be eliminated by cleaning, or the like, will occur on the surface or in the interior of the stimulable phosphor sheet obtained as a final product. In such cases, the image information at parts corresponding to the small defects will be lost in a visible image reproduced from an image signal, which has been detected from the stimulable phosphor sheet. Additionally, patterns caused to occur in the radiation image by the small defects and information in the radiation image, which information is related to a diagnosis, or the like, cannot be discriminated from each other. Therefore, an accurate judgment, e.g. an accurate diagnosis of an illness, cannot be made from the visible image. (Both the small defects and dust clinging to the surface of a stimulable phosphor sheet will hereinbelow be referred to as "defects.")